The use of manifolds in injection molding systems to transfer a melt stream from a melt source to one or more nozzles for delivering melt to one or more mold cavities is well known. Furthermore, it is well known that in many injection molding applications it is important that the manifold melt channel layout, or runner system as it is known in the art, be provided such that each cavity receives a flow of melt having the same temperature and the same shear history. Such systems can be described as “balanced”. Balancing of the manifold runner system is important in order to achieve a greater consistency, or homogeneity, of the melt stream as it is divided from a single melt stream at the manifold inlet to a plurality of manifold outlets which correspond with a plurality of mold cavities in a multi-cavity application. The result of balancing the melt stream is an overall increase in quality and uniformity of molded part being formed, when compared to parts formed in systems that are not balanced as such.
Conventional balancing of the melt stream includes designing the manifold to have geometrically matching runner layouts; that is: matching diameters, equal runner lengths, number of turns, and melt channel level changes in each melt path from the manifold inlet to a respective mold cavity. However, at times, despite having matched runner layouts, the melt stream may be different from cavity to cavity due to shear heating of the melt stream as it is forced along the melt path through the runners. More specifically, when the melt stream is forced under pressure through a bore, that is, a runner or manifold melt channel as is done in a hot runner manifold, the melt stream experiences shear in the area adjacent to the bore or melt channel wall with a corresponding localized elevation of the temperature. The result is a temperature differential across the bore or melt channel, with the center of the melt stream being cooler than the melt material closer to the bore or melt channel wall. This phenomenon is repeated at every split and/or turn of the melt stream along the melt path and may lead to an imbalance of shear-heated material between runners and subsequently between cavities of the injection molding apparatus.
Although a variety of devices and methods exist or have been proposed for addressing the need for balancing the melt delivered between cavities of a hot runner injection molding system, a need still exists for balancing or improving properties of a melt stream of moldable material flowing through a hot runner manifold so that each cavity of the system receives a relatively consistent or homogenous melt to thereby produce improved part to part consistency.